Network system

ABSTRACT

The network system includes a controller and a plurality of terminals. The controller sends a first request signal indicative of a response probability to the plurality of the terminals. Upon receiving the first request signal, each terminal sends a first response signal to the controller at the response probability indicated by the first request signal. The controller determines the total number of the terminals on the basis of the number of the received first response signals and the response probability. The controller determines a limited time period on the basis of the total number of the terminals, and sends a second request signal indicative of the limited time period to the terminals. Upon receiving the second request terminal, each terminal randomly selects a waiting time period from time periods not greater than the limited time period, and sends a second response signal to the controller after a lapse of the waiting time period.

TECHNICAL FIELD

The present invention is directed to network systems, and particularlyto a network system comprising a controller and a plurality ofterminals, each terminal sending a response signal to the controller inresponse to receiving a request signal from the controller.

BACKGROUND ART

Document 1 (JP 2006-60408 A) discloses a network system including awireless transmitter and wireless receivers. The wireless transmittertransmits a block ACK request signal to each wireless receiver. Theblock ACK request signal includes a parameter indicating a range ofrandom numbers used for determining a contention window. Each wirelessreceiver randomly selects a value of the contention window withreference to the parameters included in the block ACK request signal.Each wireless receiver sends a block ACK (response signal) after a lapseof a waiting time period starting from the time of receiving the blockACK request signal. The waiting time period is determined by use of aDIFS (Distributed Inter Frame Space) and the value of the contentionwindow.

In the network system disclosed in Document 1, each wireless receiverrandomly determines the value of the contention window. Consequently, itis possible to reduce the probability of collision between the responsesignals.

In the network system disclosed in Document 1, the maximum value of thecontention window is determined irrespective of the number of thewireless receivers. When the number of the wireless receivers isexcessive relative to the maximum value of the contention window, theprobability of collision between the response signals can not be reducedsufficiently. Therefore, the wireless transmitter may not receive theresponse signal. In contrast, when the maximum value of the contentionwindow is excessive relative to the number of the wireless receivers,the waiting time period is likely to be prolonged and communicationefficiency may become poor.

DISCLOSURE OF INVENTION

In view of the above insufficiency, the present invention has been aimedto propose a network system capable of reducing the probability ofcollision between response signals yet improving communicationefficiency.

The network system in accordance with the present invention comprises: acontroller; and a plurality of terminals connected to the controller viaa network. The controller comprises a first communication unitconfigured to communicate with the terminal via the network, a firstrequest unit, a counting unit, a terminal number determination unit, alimited time determination unit, and a second request unit. Each of theterminals comprises a second communication unit configured tocommunicate with the controller via the network, a first response unit,a waiting time determination unit, and a second response unit. The firstrequest unit is configured to send a first request signal to theplurality of the terminals via the first communication unit, the firstrequest signal including probability information indicative of apredetermined probability. The first response unit is configured to,upon receiving the first request signal via the second communicationunit, send a first response signal to the controller via the secondcommunication unit at the probability indicated by the probabilityinformation included in the first request signal. The counting unit isconfigured to count the first response signals received via the firstcommunication unit, and output the obtained number of the first responsesignals. The terminal number determination unit is configured todetermine a total number of the terminals responding to the controlleron the basis of the probability and the number of the first responsesignals obtained from the counting unit. The limited time determinationunit is configured to determine a limited time period on the basis ofthe total number of the terminals determined by the terminal numberdetermination unit. The second request unit is configured to send asecond request signal to the plurality of the terminals via the firstcommunication unit, the second request signal including limited timeinformation indicative of the limited time period determined by thelimited time determination unit. The waiting time determination unit isconfigured to, upon receiving the second request signal via the secondcommunication unit, select a waiting time period in a random manner fromtime periods not greater than the limited time period indicated by thelimited time information included in the second request signal. Thesecond response unit is configured to send a second response signal tothe controller via the second communication unit after a lapse of thewaiting time period.

In a preferred aspect, the controller further comprises a storing meansconfigured to store information derived from the number of the firstresponse signals. The controller is configured to use the informationstored in the storing means to create the second request signal and sendthe created second request signal.

In a more preferred aspect, the storing means is defined as a terminalnumber storage unit configured to store the total number of theterminals determined by the terminal number determination unit as theinformation derived from the number of the first response signals. Thelimited time determination unit is configured to determine the limitedtime period by use of the total number of the terminals stored in theterminal number storage unit.

In a further preferred aspect, the controller further comprises aterminal number update unit. The first request unit is configured tosend the first request signal to the plurality of the terminalsperiodically via the first communication unit. The terminal numberupdate unit is configured to, upon acknowledging that the terminalnumber determination unit determines the total number of the terminals,update the total number of the terminals stored in the terminal numberstorage unit with the total number of the terminals determined by theterminal number determination unit.

Alternatively, in a more preferred aspect, the storing means is definedas a limited time storage unit configured to store the limited timeperiod determined by the limited time determination unit as theinformation derived from the number of the first response signals. Thesecond request unit is configured to create the second request signalwhich includes the limited time information indicative of the limitedtime period stored in the limited time storage unit.

In a further preferred aspect, the controller further comprises alimited time update unit. The first request unit is configured to sendthe first request signal to the plurality of the terminals periodicallyvia the first communication unit. The limited time update unit isconfigured to, upon acknowledging that the limited time determinationunit determines the limited time period, update the limited time periodstored in the limited time storage unit with the limited time perioddetermined by the limited time determination unit.

In a preferred aspect, the probability indicated by the probabilityinformation is determined on the basis of a maximum number of the firstresponse signals which the first communication unit can receivesimultaneously, so as to allow said first communication unit to receivethe first response signals respectively from all of said terminalswithin a predetermined time period starting from the time of sending thefirst request signal.

In a preferred aspect, the terminal number determination unit isconfigured to determine the total number of the terminals on the basisof a quotient of the number of the first response signals obtained fromthe counting unit and the probability of the probability informationincluded in the first request signal.

In a preferred aspect, the limited time period is defined to be apredetermined range within which the first communication unit receives apredetermined number of the second response signals. The predeterminednumber is defined to be the total number of the terminals determined bythe terminal number determination unit. The limited time determinationunit is configured to calculate a quotient of the total number of theterminals determined by the terminal number determination unit and thenumber of the first response signals which the first communication unitcan receive per unit time period and determine the limited time periodon the basis of the resultant quotient.

Alternatively, in a preferred aspect, the limited time period is definedto be a predetermined range within which the first communication unitreceives a predetermined number of the second response signals. Thepredetermined number is defined to be the number of the terminals whichhave not sent the first response signal. The limited time determinationunit is configured to divide a difference by the number of the firstresponse signals which the first communication unit can receive per unittime period to determine the limited time period. The difference isdefined as a difference between the total number of the terminalsdetermined by the terminal number determination unit and the number ofthe first response signals counted by the counting unit. Each of theterminals is configured to send no second response signal once aftersending the first response signal to the controller.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a diagram illustrating a configuration of the network systemof the first embodiment,

FIG. 2 is a diagram illustrating a configuration of a request signal,

FIG. 3 is a diagram illustrating a configuration of a response signal,

FIG. 4 is a diagram illustrating a configuration of a first requestsignal of the network system in accordance with the first embodiment,

FIG. 5 is a diagram illustrating a configuration of a second requestsignal of the network system in accordance with the first embodiment,

FIG. 6 is a sequence diagram illustrating operation of the networksystem of the first embodiment,

FIG. 7 is a diagram illustrating a configuration of a modification ofthe network system of the first embodiment,

FIG. 8 is a diagram illustrating a configuration of the network systemof the second embodiment, and

FIG. 9 is a diagram illustrating a configuration of the second requestsignal of the network system in accordance with the second embodiment.

BEST MODE FOR CARRYING OUT THE INVENTION First Embodiment

FIG. 1 shows a configuration of the network system of the presentembodiment. The network system of the present embodiment includes acontroller (multicast terminal device) 10 and a plurality of terminals20 connected to the controller 10 via a network 30. In the presentnetwork system, plural terminal devices (the controller 10 and terminals20) are connected to the network 30, and communicate with each other.For example, each of the controller 10 and terminals 20 comprises amicrocomputer including such as a memory, a processor, and an interfaceadapted in use to be connected to the network 30. Besides, the network30 may be a wired network or a wireless network.

The controller 10 is configured to monitor and control each of theterminals 20. In more detail, the controller 10 sends a request signal(response requesting command) to each of the terminals 20. The requestsignal is a multicast packet such as a request signal for controllingthe terminal 20, a request signal for obtaining a status of the terminal20, and a request signal for changing setting values of the terminal 20.Upon receiving the request signal, the terminal 20 performs processingcorresponding to a content of the received request signal. Further, theterminal 20 sends a response signal to the controller 10. The responsesignal is such as a control completion response signal indicatingcompletion of control, a status announcing response signal includingstatus data, and a setting completion response signal indicatingcompletion of change of settings. The controller 10 receives theresponse signals from each terminal 20.

The controller 10 includes a first communication unit 101, a firstfunction unit 102, and a first control unit 103. Further, the controller10 includes a probability setting unit 104, a first request unit 105 forsending a request signal 40 (first request signal 41), a counting unit106, a terminal number determination unit 107, a terminal number updateunit 108, a terminal number storage unit 109, a limited timedetermination unit 110, and a second request unit 111 for sending arequest signal 40 (second request signal 42).

The first communication unit 101 is a network interface forcommunicating with the terminals 20 via the network 30. The firstcommunication unit 101 is configured to store address informationpeculiar to the controller 10. The first communication unit 101establishes network communication by use of the stored addressinformation. For example, the address information is an IP address and aphysical address (MAC address).

The first function unit 102 is configured to execute processing(operation) for a service (function of the controller 10) to be providedby the controller 10. In the present embodiment, the first function unit102 communicates with the terminals 20 via the network 30, therebymonitoring and controlling the terminals 20.

The terminal 20 may be lighting apparatus or air conditioning apparatus.The lighting apparatus is configured to turn on and off lightingequipment and to dim the lighting equipment. The air conditioningapparatus is configured to drive air conditioning equipment.

Each of the terminals 20 includes a second communication unit 201, asecond function unit 202, and a second control unit 203. Each of theterminals 20 further includes a first response unit 204 for sending aresponse signal 50 (first response signal 51), a waiting timedetermination unit 205, and a second response unit 206 for sending aresponse signal 50 (second response signal 52).

The second communication unit 201 is a network interface forcommunicating with the controller 10 via the network 30. The secondcommunication unit 201 is configured to store address informationpeculiar to the terminal 20. The second communication unit 201establishes the network communication by use of the stored addressinformation. For example, the address information is an IP address and aphysical address (MAC address).

The second function unit 202 is configured to execute processing(operation) for a service (function of the terminal 20) to be providedby the terminal 20. The second function unit 202 is a target to becontrolled or monitored by the controller 10. When the terminal 20 islighting apparatus, the second function unit 202 is a lighting means forturning on and off and diming a lighting load (lighting equipment). Whenthe terminal 20 is air conditioning apparatus, the second function unit202 is an air conditioner controlling means for controlling airconditioning equipment to regulate a room temperature.

For the purpose of giving an object server function to the first andsecond control units 103 and 203, each of the first and second controlunits 103 and 203 is provided with a software module including objectswhich are used to perform processing for providing a service.

Each object has one or more interfaces (input output definition). Theinterface defines control information or monitoring information (e.g., avariable, function, event information, and a combination thereof)corresponding to the service to be provided by the function unit 102 or202 (the service to be provided by the controller 10 or the terminal20).

The object has an object unique identifier (referred to as “OID”, ifnecessary) uniquely. The interface has an interface identifier (referredto as “IID”, if necessary) uniquely. There is a specific identifierwhich is allocated to a combination of the object and the interface. Inthe following, a content of the specific identifier is expressed as[OID+IID].

The first control unit 103 functions as an object controlling means anda response signal sending means. The second control unit 203 functionsas a request signal sending means configured to create the requestsignal using the specific identifier and send the created requestsignal.

The object controlling means is configured to, upon receiving therequest signal using the specific identifier, operate the object inresponse to the contents of the request signal to provide the controlinformation to the function unit 102. In addition, the objectcontrolling means is configured to perform processing of obtaining themonitoring information from the function unit 102. The objectcontrolling means is configured to perform processing of obtaininginformation regarding the object. The object controlling means isconfigured to perform processing of providing a function for indicatingoperation to the function unit 102. The object controlling means isconfigured to perform processing of obtaining the variable indicatingthe current status of the function unit 102. The object controllingmeans is configured to perform processing of obtaining the eventinformation indicating change in the status of the function unit 102.

The response signal sending means is configured to send the responsesignal to the terminal device which is the sender of the request signal.The response signal includes a result of the processing executed by theobject controlling means.

FIG. 2 shows a configuration of the request signal 40. The requestsignal 40 includes an operation code 401, the interface identifier 402,and the object unique identifier 403. The operation code 401 denotes thetype of the response requesting command. The interface identifier 402denotes the service (function) provided by the terminal device. Theobject unique identifier 403 denotes the control target or themonitoring target. Further, the request signal 40 may include aparameter 404 depending on the type of the operation code 401. When theterminal 20 has the object having the specific identifier which isdefined by the interface identifier 402 and the object unique identifier403 included in the request signal 40, the terminal 20 receives therequest signal 40 via the second communication unit 201.

For example, the operation code 401 includes “Watch”, “Get”, “Set”, and“Invoke”. The operation code “Watch” is used to request a search fordetermining whether or not the object having the OID and IID designatedby the request signal 40 is present in the network 30. The operationcode “Get” is used to request to obtain a value of the object having theOID and IID designated by the request signal 40. The operation code“Set” is used to request to set a value to the object having the OID andIID designated by the request signal 40. The operation code “Invoke” isused to request to execute the object having the OID and IID designatedby the request signal 40 to provide the control information to thefunction unit 202.

A wildcard “*” is available in the request signal 40. The wildcarddenotes any of OIDs or IIDs. For example, when the controller 10 sendsthe request signal having the object unique identifier 403 of “*”, theinterface identifier 402 of “*”, and the operation code 401 of “Watch”,all of the terminals 20 receives the request signal, and sends theresponse signal including the combination of the object and theinterface to the controller 10.

For example, the controller 10 sends the request signal in which onlythe interface identifier 402 is the wildcard “*” and the operation code401 denotes the “Watch”. Upon receiving the request signal, the terminal20 sends, to the controller 10, the response signal including thecombination of the OID designated by the request signal and theinterface of the OID designated by the request signal.

For example, the controller 10 sends the request signal in which onlythe object unique identifier 403 is the wildcard “*” and the operationcode 401 denotes the “Watch”. Upon receiving the request signal, theterminal 20 sends, to the controller 10, the response signal includingthe combination of the IID designated by the request signal and the OIDincluding the IID designated by the request signal.

In the present embodiment, using the wildcard “*” as the object uniqueidentifier 403 of the request signal 40 enables multicast communication.

FIG. 3 illustrates the response signal 50. The response signal 50includes an operation code 501 and terminal identification information502. The operation code 501 denotes the type of the response signal 50.The terminal identification information 502 denotes the identificationinformation (in the present embodiment, the address of the terminal 20)of the terminal 20 being the sender of the response signal. Further, theresponse signal 50 may include a specific parameter 503 depending on thetype of the operation code 501.

For example, upon receiving the request signal 40 having the operationcode 401 of “Watch”, the terminal 20 sends the response signal 50 havingthe operation code 501 of “Provide”. In this instance, the responsesignal 50 includes the parameter 503 which denotes the combination ofthe object unique identifier and the interface identifier which aredesignated by the request signal 40.

Besides, the plural objects may have the interface (i.e., the interfacehaving the same IID) which has the same definition corresponding to theservice provided by the terminal device.

An OSI 7-layer model is used as the protocol for the network system ofthe present embodiment. As an application layer of the uppermost layer,a unique object access protocol (OAP) is used to give and receive theinformation such as the variable, the function and the event informationbetween software modules of the controller 10 and the terminal 20.

When the first control unit 103 sends the request signal 40 to theterminals 20, at first the first control unit 103 provides a request ofsending the first request signal 41 to the first request unit 105, andsubsequently provides a request of sending the second request signal 42to the second request unit 111. The first and second request signalsinclude the same interface identifier 402 and object identifier 403.

The first request unit 105 is configured to, in response to receivingthe request of sending the first request signal 41, send the firstrequest signal 41 to the plurality of the terminals 20 via the firstcommunication unit 101. The first request unit 105 is configured to sendthe first request signal 41 to the plurality of the terminals 20 via thefirst communication unit 101 periodically.

The first request signal 41 includes, as shown in FIG. 4, the operationcode 401, the interface identifier 402, the object unique identifier403, and probability information 405. The first request signal 41 mayinclude the specific parameter 404 depending on the type of theoperation code 401. The probability information indicates apredetermined probability (response probability).

The probability setting unit 104 is an input device for inputting theresponse probability. The response probability defines a probabilitythat the terminal 20 sends the first response signal 51 in response toreceiving the first request signal 41. In other words, the responseprobability defines a probability that upon receiving the first requestsignal 41 the terminal 20 performs operation corresponding to theoperation code 401 of the first request signal 41 and sends the firstresponse signal 51 to the controller 10. The response probability is inthe range of 0 to 100 [%]. When the response probability is 100 [%], theterminal 20 is sure to send the first response signal 51. When theresponse probability is 50 [%], the terminal 20 sends the first responsesignal 51 at the probability of 50 [%].

The response probability is determined on the basis of a maximum number(maximum simultaneous reception number) of the first response signals 51which the first communication unit 101 can receive simultaneously, so asto allow the first communication unit 101 to receive the first responsesignals 51 respectively from all of the terminals 20 within apredetermined time period starting from the time of sending the firstrequest signal 41. The maximum simultaneous reception number varies inaccordance with access control performed in a communication transport(transport) of the network 30. For example, in CSMA/CA communication,the maximum simultaneous reception number varies in accordance withaccuracy of carrier sense, a backoff method for the carrier sense, alength of backoff time, or the retransmission number.

The response probability is set to a sufficiently-small value (e.g.,10%) in consideration of the communication transport of the network 30,in order to allow the controller 10 to receive all of the first responsesignals 51 respectively sent from the terminals without causing nopacket loss in transmitting the first response signals 51.

The first response unit 204 is configured to, upon receiving the firstrequest signal 41 via the second communication unit 201, send the firstresponse signal 51 to the controller 10 via the second communicationunit 201 at the response probability indicated by the probabilityinformation included in the first request signal 41. The first responseunit 204 functions as the object controlling means and the responsesignal sending means. The first response signal 51 has the substantiallysame configuration as that of the response signal 50.

The first response unit 204 includes a first random number creationmodule 2041, a response judgment module 2042, and a first executionmodule 2043.

The first random number creation module 2041 is configured to create arandom number within the range of 0 to 100. The first random numbercreation module 2041 provides the resultant random number to theresponse judgment module 2042.

The response judgment number 2042 is configured to, upon receiving thefirst request signal 41 via the second communication unit 201, requestthe first random number creation module 2041 to create the randomnumber. The response judgment module 2042 is configured to, uponobtaining the random number from the first random number creation module2041, compare the random number obtained from the first random numbercreation module 2041 with the response probability indicated by theprobability information 405 included in the first request signal 41. Theresponse judgment module 2042 determines to send the first responsesignal 51 when the random number is less than the response probability.The response judgment module 2042 determines not to send the firstresponse signal 51 when the random number is not less than the responseprobability.

When the response judgment module 2042 determines to send the firstresponse signal 51, the first execution module 2043 operates the objectdesignated by the operation command 401 of the first request signal 41,and sends the first response signal 51. When the response judgmentmodule 2042 determines not to send the first response signal 51, thefirst execution module 2043 does not operate the object designated bythe operation command 401 of the first request signal 41 and send thefirst response signal 51, too.

The counting unit 106 is configured to count the first response signals51 which the first communication unit 101 has received. The countingunit 106 counts the first response signals 51 received by the firstcommunication unit 101 until a predetermined time period elapses fromthe time at which the first communication unit 101 sends the firstrequest signal 41. After a lapse of the predetermined time period, thecounting unit 106 provides the number of the first response signals 51to the terminal number determination unit 107.

The terminal number determination unit 107 is configured to determine amaximum number (total number) of the terminals 20 responding to thecontroller 10 on the basis of the response probability indicated by theprobability information included in the first request signal 41 and thenumber of the first response signals 51 obtained from the counting unit106. In other words, the terminal number determination unit 107determines the maximum number of the terminals 20 which send the firstresponse signal 51 to the controller 10 in response to receiving thefirst request signal 41. The terminal number determination unit 107 isconfigured to determine the total number of the terminals 20 on thebasis of a quotient of the number of the first response signals 51obtained from the counting unit 106 and the response probability of theprobability information included in the first request signal 41. In thepresent embodiment, the unit of the response probability is [%]. Theterminal number determination unit 107 calculates the total number ofthe terminals 20 by multiplying 100 by the quotient obtained by dividingthe number of the first response signals 51 obtained from the countingunit 106 by the response probability of the probability informationincluded in the first request signal 41. For example, when the responseprobability is 10 [%] and the number of the first response signals 51 is10, the total number of the terminals 20 is 100.

The terminal number storage unit 109 is used for storing the totalnumber of the terminals 20 determined by the terminal numberdetermination unit 107.

Upon determination of the total number of the terminals 20 at theterminal number determination unit 107, the terminal number update unit108 responds to store the total number of the terminals 20 in theterminal number storage unit 109. Further, the terminal number updateunit 108 is configured to, when the terminal number determination unit107 newly determines the total number of the terminals 20, update thetotal number of the terminals 20 in the terminal number storage unit 109in match with the newly determined total number of the terminals 20.

The limited time determination unit 110 is configured to determine alimited time period on the basis of the total number of the terminalsdetermined by the terminal number determination unit 107. The limitedtime determination unit 110 is configured to determine the limited timeperiod by use of the total number of the terminals 20 thus stored in theterminal number storage unit 109 once after the terminal number storageunit 109 has stored the total number of the terminals 20.

The limited time period is defined to be a predetermined range withinwhich the first communication unit 101 receives a predetermined numberof the second response signals 52. Especially, the limited time periodis preferred to be a minimum time period within which the firstcommunication unit 101 receives a predetermined number of the secondresponse signals 52. In the present embodiment, the limited time periodis determined as a maximum time period within which all the terminals 20in the number estimated by the terminal number determination unit 107can send the second response signal 52 to the controller 10. In otherwords, the limited time period is defined as a time period within whichthe controller 10 receives the second response signals 52 respectivelysent from all of the terminals 20 in the number equivalent to the totalnumber of the terminals 20 estimated by the terminal numberdetermination unit 107. The limited time period is determined on thebasis of the transport used by the network 30. The communication speedof the transport is important for determining the limited time period.As described below, the terminal 20 determines a waiting time period ina random manner. The waiting time period varies from 0 to the limitedtime period theoretically. However, practically, the waiting time perioddoes not always vary theoretically. Therefore, the limited time periodneed have an enough range such that the plural terminals 20 have thedifferent waiting time periods. In addition, the limited time period ispredetermined not to be less than a time period within which the targetterminal 20 completes the communication at the speed identical to thecommunication speed of the transport.

In the present embodiment, the above predetermined number is the totalnumber of the terminals 20 determined by the terminal numberdetermination unit 107. Thus, in the present embodiment, the limitedtime determination unit 110 divides the total number of the terminals 20determined by the terminal number determination unit 107 by the maximumnumber of the first response signals 51 which the first communicationunit 101 can receive per unit time, thereby determining the limited timeperiod. For example, when the maximum number of the first responsesignals 51 which the first communication unit 101 can receive per unittime (e.g., 1 second) is 10 (the communication transport used in thenetwork 30 can process ten terminals per one second), the limited timeperiod is a quotient (=10 [sec]) obtained by dividing the total number(=100) of the terminals 20 determined by the terminal numberdetermination unit 107 by the maximum number (=10) of the first responsesignals 51 which the first communication unit 101 can receive per unittime.

The limited time determination unit 110 may store a data tableindicative of a correspondence relation between the total number of theterminals 20 and the limited time period. In this instance, the limitedtime determination unit 110 may read out from the above data table thelimited time period corresponding to the total number of the terminals20 obtained from the terminal number determination unit 107.

The second request unit 111 is configured to control the firstcommunication unit 101 so as to send the second request signal 42 (seeFIG. 5) to the plurality of the terminal 20 via the first communicationunit 101. In response to receiving the request of sending the secondrequest signal 42, the second request unit 111 requests the limited timedetermination unit 110 to determine the limited time period. The limitedtime determination unit 110 determines the limited time period on thebasis of the total number of the terminals 20 stored in the terminalnumber storage unit 109 and provides the resultant limited time periodto the second request unit 111 in response to the request from thesecond request unit 111.

The second request signal 42 includes, as shown in FIG. 5, the operationcode 401, the interface identifier 402, the object unique identifier403, and limited time information 406. The limited time information 406indicates the limited time period determined by the limited timedetermination unit 110. The second request signal 42 may include thespecific parameter 404 depending on the type of the operation code 401.The second request signal 42 and the first request signal 41 have thesame operation code 401, interface identifier 402, and object uniqueidentifier 403. In other words, the second request unit 113 sends thesecond request signal 42 to the terminal 20 which sends the firstresponse signal 51 to the controller 10 in response to the first requestsignal 41.

The waiting time determination unit 205 is configured to, in response toreceiving the second request signal 42 via the second communication unit201, select randomly the waiting time period from time periods less thanthe limited time period indicated by the limited time informationincluded in the second request signal 42. The waiting time determinationunit 205 includes a second random number creation module 2051 and awaiting time calculation module 2052.

The second random number creation module 2051 is configured to create arandom number within the range of 0 to 100. The second random numbercreation module 2051 provides the resultant random number to the waitingtime calculation module 2052. Besides, the first random number creationmodule 2041 may be use as the second random number creation module 2051.In this instance, the second random number creation module 2051 can beomitted.

The waiting time calculation module 2052 is configured to request thesecond random number creation module 2051 to create the random numberupon receiving the second request signal 42 via the second communicationunit 201. The waiting time calculation module 2052 is configured tocalculate the waiting time period upon obtaining the random number fromthe second random number creation module 2051. The waiting time periodis determined on the basis of a quotient obtained by dividing a productby 100, the product being obtained by multiplying the limited timeperiod of the limited time information included in the second requestsignal 42 by the random number (obtained from the second random numbercreation module 2051). Consequently, the waiting time determination unit205 determines the waiting time period randomly within the range of 0 tothe limited time period [sec].

The second response unit 206 is configured to control the secondcommunication unit 201 so as to send the second response signal 52 tothe controller 10 via the second communication unit 201 after a lapse ofthe waiting time period. The second request unit 206 functions as theobject controlling means and the response signal sending means. Thesecond response unit 206 includes a timer module 2061 and a secondexecution module 2062. Upon obtaining the waiting time period from thewaiting time calculation module 2052, the timer module 2061 startstiming processing of measuring the waiting time period. When the waitingtime period elapses, the timer module 2061 notifies the second executionmodule 2062 of the lapse of the waiting time period. Upon being notifiedof the lapse of the waiting time period, the second execution module2062 operates the object designated by the operation command 401 of thesecond request signal 42, and sends the second response signal 52. Thesecond response signal 52 has the substantially same configuration asthat of the response signal 50.

Next, an explanation using FIG. 6 is made to operation of the networksystem of the present embodiment. In the following explanation, in orderto distinguish between the plural terminals 20, the terminals 20 arerespectively designated by use of reference numerals 21, 22, . . . , 2n, if necessary.

The terminal 21 includes the object having the object unique identifier“OID1”. The object having “OID1” includes interfaces respectively havingthe interface identifiers “IID6” and “IID7”.

The terminal 22 includes the object having the object unique identifier“OID2”. The object having “OID2” includes interfaces respectively havingthe interface identifiers “IID3” and “IID7”.

The other terminals 20 include the object containing the interfacehaving the interface identifier “IID7”.

As to the controller 10, the first request unit 105 sends periodicallythe first request signal 41 to the plurality of the terminals 20 via thefirst communication unit 101 (S10). For example, the first requestsignal 41 includes the operation code 401 of “Watch”, the object uniqueidentifier 403 of “*”, the interface identifier 402 of “IID7”, and theprobability information indicating the response probability of “10%”.

Only the terminal 20 including the interface having the interfaceidentifier “IID7” receives the first request signal 41. In the terminal20 which has received the first request signal 41, the first responseunit 204 controls the second communication unit 201 at the responseprobability indicated by the probability information of the firstrequest signal 41. Thus, the first response signal 51 is sent to thecontroller 10 at the response probability indicated by the probabilityinformation of the first request signal 41 (S20).

As to the first response signal 51, the operation code 501 is “Provide”.The terminal identification information 502 denotes the address of theterminal 20 being the sender of the first response signal 51. Theparameter 503 is the combination of the interface identifier “IID7” andthe object unique identifier associated with the interface identifier“IID7”.

For example, the terminal 21 transmits the first response signal 51including the parameter 503 indicating the combination of the interfaceidentifier “IID7” and the object unique identifier “OID1”. The terminal22 transmits the first response signal 51 including the parameter 503indicating the combination of the interface identifier “IID7” and theobject unique identifier “OID2”.

It is supposed that the maximum number of the terminals 20 sending thefirst response signal 51 in response to receiving the first requestsignal 41 is 100 (that is, the number of the terminals including theinterface having the interface identifier “IID7” is 100). Theprobability information of the first request signal 41 indicates theresponse probability of 10%. Theoretically, the ten terminals 20 sendthe first response signal 51.

In the controller 10, the counting unit 106 counts the first responsesignals 51 which the first communication unit 101 has received. Theterminal number determination unit 107 determines the maximum number ofthe terminals 20 sending the first response signal 51 in response toreceiving the first request signal 41 on the basis of the number of thefirst response signals 51 obtained from the counting unit 106 and theresponse probability. In other words, the terminal number determinationunit 107 determines the maximum number of the terminals 20 including theinterface having the interface identifier “IID7”. When the ten terminals20 send the first response signal 51, the first communication unit 101receives the ten first response signals 51. The response probability is“10%”. Thus, the terminal number determination unit 107 divides thenumber (=10) of the first response signals 51 received at the firstcommunication unit 101 by the response probability (=10%) to obtain aquotient (=1), and multiplies the resultant quotient (=1) by 100 toobtain the total number of the terminals 20.

Upon receiving the request of sending the second request signal 42, thesecond request unit 111 requests the limited time determination unit 110to determine the limited time period.

In response to the request from the second request unit 111, the limitedtime determination unit 110 determines the limited time period T10 onthe basis of the total number of the terminals 20 stored in the terminalnumber storage unit 109 and provides the limited time period T10 to thesecond request unit 111. When the total number of the terminals 20determined by the terminal number determination unit 107 is 100 and themaximum number of the first response signals 51 which the firstcommunication unit 101 can receive per unit time is 10, the limited timeperiod T10 is 10 [sec].

The second request unit 111 sends the second request signal 42 to theplurality of the terminals 20 via the first communication unit 101(S30). The second request signal 42 includes the same operation code 401(=“Watch”), object unique identifier 403 (=“*”), and interfaceidentifier 402 (=“IID7”) as those of the first request signal 41.Further, the second request signal 42 includes the limited timeinformation 406 indicative of the limited time period determined by thelimited time determination unit 110.

Only the terminal 20 including the interface having the interfaceidentifier “IID7” receives the second request signal 42. In the terminal20 which has received the second request signal 42, the waiting timedetermination unit 205 determines the waiting time period on the basisof the limited time period T10 indicated by the limited time informationof the second request signal 42. The second response unit 206 sends thesecond response signal 52 via the second communication unit 201 after alapse of the waiting time period determined by the waiting timedetermination unit 205 (S40). As to the second response signal 52, theoperation code 501 is “Provide”. The terminal identification informationdenotes the address of the terminal 20 being the sender of the secondresponse signal 52. The parameter 503 is the combination of theinterface identifier “IID7” and the object unique identifier associatedwith the interface identifier “IID7”.

As described in the above, the network system of the present embodimentincludes the controller 10 and the plurality of the terminals 20connected to the controller 10 via the network 30. The controller 10includes the first communication unit 101 configured to communicate withthe terminal 20 via the network 30, the first request unit 105, thecounting unit 106, the terminal number determination unit 107, thelimited time determination unit 110, and the second request unit 111.Each of the terminals 20 includes the second communication unit 201configured to communicate with the controller 10 via the network 30, thefirst response unit 204, the waiting time determination unit 205, andthe second response unit 206. The first request unit 105 is configuredto send the first request signal 41 to the plurality of the terminals 20via the first communication unit 101, the first request signal 41including probability information indicative of the predeterminedresponse probability. The first response unit 204 is configured to, uponreceiving the first request signal 41 via the second communication unit201, send the first response signal 51 to the controller 10 via thesecond communication unit 201 at the response probability indicated bythe probability information included in the first request signal 41. Thecounting unit 106 is configured to count the first response signals 51received via the first communication unit 101, and output the obtainednumber of the first response signals 51. The terminal numberdetermination unit 107 is configured to determine the total number ofthe terminals 20 responding to the controller 10 on the basis of theresponse probability and the number of the first response signals 51obtained from the counting unit 106. The limited time determination unit110 is configured to determine the limited time period on the basis ofthe total number of the terminals 20 determined by the terminal numberdetermination unit 107. The second request unit 111 is configured tosend the second request signal 42 to the plurality of the terminals 20via the first communication unit 101, the second request signal 42including limited time information indicative of the limited time perioddetermined by the limited time determination unit 110. The waiting timedetermination unit 205 is configured to, upon receiving the secondrequest signal 42 via the second communication unit 201, select thewaiting time period in a random manner from time periods not greaterthan the limited time period indicated by the limited time informationincluded in the second request signal 42. The second response unit 206is configured to send the second response signal 52 to the controller 10via the second communication unit 201 after the waiting time periodelapses.

In accordance with the network system of the present embodiment, each ofthe terminals 20 is responsive to the second request signal 42 multicastfrom the controller 10 for sending the second response signal 52 at atiming which is randomly determined at each of the terminals 20.Ideally, the timings determined respectively for the terminals 20 aredistributed evenly within the limited time range provided by the limitedtime determination unit 110 (e.g., the range of 0 to 10 [sec]).Therefore, even when the network 30 employs the communication transportproviding relatively low speed communication, it is possible to reducethe packet loss otherwise caused by collision between the secondresponse signals 52. All of the second response signals 52 are sentbefore a lapse of the limited time period (e.g., 10 [sec]) determined incorrespondence to the number of the terminals 20. Therefore, the limitedtime period can be prevented from becoming too short or long in relationto the number of the terminals 20. Thus, the network system of thepresent embodiment can prevents the collision between the responsesignals, yet improving communication efficiency.

In the network system of the present embodiment, the responseprobability is determined on the basis of the basis of the maximumnumber of the first response signals 51 which the first communicationunit 101 can receive simultaneously, so as to allow the firstcommunication unit 101 to receive the first response signals 51respectively from all of the terminals 20 within the predetermined timeperiod starting from the time of sending the first request signal 41.

Therefore, the controller 10 successfully receives the first responsesignals 51 respectively sent from the terminals 20 responding to thecontroller 10 without occurrence of the packet loss of the firstresponse signal 51.

In the network system of the present embodiment, the terminal numberdetermination unit 107 is configured to determine the total number ofthe terminals 20 on the basis of the quotient of the number of the firstresponse signals 51 obtained from the counting unit 106 and the responseprobability of the probability information included in the first requestsignal 41.

Therefore, the total number of the terminals 20 can be calculated withhigh accuracy.

In the network system of the present embodiment, the limited time periodis defined to be a predetermined range within which the firstcommunication unit 101 receives a predetermined number of the secondresponse signals 52. The predetermined number is defined to be the totalnumber of the terminals 20 determined by the terminal numberdetermination unit 107. The limited time determination unit isconfigured to calculate the quotient of the total number of theterminals 20 determined by the terminal number determination unit 107and the number of the first response signals 51 which the firstcommunication unit 101 can receive per unit time period and determinethe limited time period on the basis of the resultant quotient.

Therefore, the limited time period can be set to a minimum time periodwithin which the controller performs communication processes in thenumber identical to the total number of the terminals 20.

In the network system of the present embodiment, the controller 10includes the terminal number storage unit 109 configured to store thetotal number of the terminals 20 determined by the terminal numberdetermination unit 107. The limited time determination unit 110 isconfigured to determine the limited time period by use of the totalnumber of the terminals 20 stored in the terminal number storage unit109 once after the terminal number storage unit 109 stores the totalnumber of the terminals 20. Therefore, the second request unit 111generates the second request signal 42 including the limited timeinformation indicative of the limited time period determined on thebasis of the total number of the terminals 20 stored in the terminalnumber storage unit 109. The first communication unit 101 sends thesecond request signal 42 prepared in the aforementioned manner.

In other words, the controller 10 includes a storing means (terminalnumber storage unit 109) configured to store information (the totalnumber of the terminals 20) derived from the number of the firstresponse signals 51 which the first communication unit 101 has received.The controller 10 is configured to use the information stored in thestoring means to create the second request signal 42 and send thecreated second request signal 42.

The controller 10 includes the terminal number storage unit 109configured to store the total number of the terminals 20 responding tothe first request signal 41, as the information derived from the numberof the first response signals 51 which has been received by the firstcommunication unit 101. Accordingly, once after the terminal numberstorage unit 109 has stored the total number of the terminals 20, thereis no need for determining the total number of the terminals 20responding to the first request signal 41, and the first request signal41 need not be sent. Thus, it is unnecessary to send a set of the firstand second request signals 41 and 42 every time for the purpose ofobtaining the second response signal 52 from the terminal 20.Consequently, it is possible to reduce the communication traffic, and toshorten the transmission processing. Especially, the network system ofthe present embodiment is suitable for a network system which seeslittle change in the number of the terminals 20.

The first request signal 41 includes the specific identifier [OID+IID].Therefore, only the terminal 20 having the object corresponding to thespecific identifier sends the first response signal 51. Thus, theterminal number storage unit 109 stores the total number of theterminals 20, for each class of IID of the object (for each classdefined by the specific identifier). For example, the terminal numberstorage unit 109 stores the total number “N1” regarding IID1, the totalnumber “N2” regarding IID2, the total number “N3” regarding IID3, andthe total number “N4” regarding IID4. The limited time determinationunit 110 can determine the appropriate limited time T10 for each classdefined by the specific identifier.

The controller 10 includes the terminal number update unit 108. Thefirst request unit 105 is configured to send the first request signal 41to the plurality of the terminals 20 periodically via the firstcommunication unit 101. The terminal number update unit 108 isconfigured to, when the terminal number determination unit 107determines the total number of the terminals 20, update the total numberof the terminals 20 stored in the terminal number storage unit 109 insuch a manner that the terminal number storage unit 109 stores thelatest total number of the terminals 20 determined by the terminalnumber determination unit 107.

In other words, the controller 10 updates at regular intervals the totalnumber of the terminals 20 stored in the terminal number storage unit110. Therefore, the controller is enabled to adapt itself to the changein the configuration (the number of the terminals 20) of the networksystem. Moreover, the limited time period can be set to an appropriatetime period depending on the number of the terminals 20.

FIG. 7 illustrates the network system according to a modification of thepresent embodiment. The network system of the modification includes thecontroller 10A and the plurality of terminals 20 connected to thecontroller 10A via the network 30. Components common to the networksystem of the modification and the network system of the presentembodiment are designated by same reference numerals, and noexplanations thereof are deemed necessary.

The controller 10A includes the first communication unit 101, the firstfunction unit 102, and the first control unit 103. Further, thecontroller 10A includes the probability setting unit 104, the firstrequest unit 105, the counting unit 106, the terminal numberdetermination unit 107, the limited time determination unit 110, alimited time update unit 112, a limited time storage unit 113, and thesecond request unit 111A.

The limited time storage unit 113 is used for storing the limited timedetermined by the limited time determination unit 110.

Upon determination of the limited time period at the limited timedetermination unit 110, the limited time update unit 112 responds tostore the limited time period in the limited time storage unit 113.Further, the limited time update unit 112 is configured to, when thelimited time determination unit 110 newly determines the limited timeperiod, update the limited time period in the limited time storage unit113 in match with the newly determined limited time period.

Upon receiving the request of sending the second request signal 42, thesecond request unit 111A reads out the limited time period from thelimited time storage unit 113. In other words, the second request unit111A is configured to, once after the limited time storage unit 113 hasstored the limited time period, transmit to the plurality of theterminals 20 via the first communication unit 101 the second requestsignal 42 including the limited time information 406 indicative of thelimited time period stored in the limited time storage unit 113.

According to the network system of the present embodiment, thecontroller 10A includes the limited time storage unit 113 configured tostore the limited time period determined by the limited timedetermination unit 110, as the information derived from the number ofthe first response signals 51 which the first communication unit 101 hasreceived. The second request unit 111A creates the second request signal42 which includes the limited time information indicative of the limitedtime period stored in the limited time storage unit 113 once after thelimited time storage unit 113 has stored the limited time period. Thefirst communication unit 101 sends the second request signal 42 preparedin the aforementioned manner.

As described in the above, according to the network system of themodification of the present embodiment, the controller 10A includes thestoring means (limited time storage unit 113) configured to store theinformation (limited time period) derived from the number of the firstresponse signals 51 which the first communication unit 101 has received.The controller 10A is configured to create the second request signal 42on the basis of the information (limited time period) stored in thestoring means (limited time storage unit 113) and send the secondrequest signal 42 once after the storing means (limited time storageunit 113) has stored the information (limited time period).

Accordingly, once after the limited time storage unit 113 has stored thelimited time period, there is no need for determining the limited timeperiod, and the first request signal 41 need not be sent. Thus, it isunnecessary to send a set of the first and second request signals 41 and42 every time for the purpose of obtaining the second response signal 52from the terminal 20. Consequently, it is possible to reduce thecommunication traffic, and to shorten the transmission processing.Especially, the network system of the present embodiment is suitable fora network system which sees little change in the number of the terminals20.

The first request signal 41 includes the specific identifier [OID+IID].Therefore, only the terminal 20 having the object corresponding to thespecific identifier sends the first response signal 51. Thus, thelimited time storage unit 113 stores the limited time period, for eachclass of IID of the object (for each class defined by the specificidentifier).

The controller 10A includes the limited time update unit 112. The firstrequest unit 105 is configured to send the first request signal 41 tothe plurality of the terminals 20 periodically via the firstcommunication unit 101. The limited time update unit 112 is configuredto, when the limited time determination unit 110 determines the limitedtime period, update the limited time period stored in the limited timestorage unit 113 with the limited time period determined by the limitedtime determination unit 110.

In other words, the controller 10A updates at regular intervals thelimited time period stored in the limited time storage unit 113.Therefore, the controller is enabled to adapt itself to the change inthe configuration (the number of the terminals 20) of the networksystem. Moreover, the limited time period can be set to an appropriatetime period depending on the number of the terminals 20.

Second Embodiment

FIG. 8 illustrates the network system of the present embodiment. Thenetwork system of the present embodiment includes the controller 10B andthe plurality of terminals 20B connected to the controller 10B via thenetwork 30. Components common to the network systems of the first andpresent embodiments are designated by same reference numerals, and noexplanations thereof are deemed necessary.

The controller 10B includes the first communication unit 101, the firstfunction unit 102, and the first control unit 103, the probabilitysetting unit 104, the first request unit 105B, the counting unit 106B,the terminal number determination unit 107B, the limited timedetermination unit 110B, the second request unit 111B, and a respondingterminal storage unit 114.

The terminal 20B includes the second communication unit 201, the secondfunction unit 202, the second control unit 203, the first response unit204, the waiting time determination unit 205B, the second response unit206, and a transmission check unit 207.

The first request unit 105B is configured to, in response to receivingthe request of sending the first request signal 41, send the firstrequest signal 41 to the plurality of the terminals 20B via the firstcommunication unit 101.

The counting unit 106B is configured to count the first response signals51 received by the first communication unit 101 until a predeterminedtime period (reception time period) elapses from the time at which thefirst communication unit 101 has sent the first request signal 41. Thecounting unit 106B provides the counted number of the first responsesignals 51 to the terminal number determination unit 107B and thelimited time determination unit 110B.

The terminal number determination unit 107B is configured to determinethe maximum number (total number) of the terminals 20B responding to thecontroller 10B on the basis of the response probability indicated by theprobability information 405 included in the first request signal 41 andthe number of the first response signals 51 obtained from the countingunit 106B. The terminal number determination unit 107B provides thetotal number of the terminals 20B to the limited time determination unit110B.

The limited time determination unit 110B determines the limited timeperiod on the basis of the total number of the terminals 20B obtainedfrom the terminal number determination unit 107B. The limited timedetermination unit 110B is configured to calculate a difference of thetotal number of the terminals 20B determined by the terminal numberdetermination unit 107B and the number of the first response signals 51counted by the counting unit 106B and calculate a quotient of theresultant difference and the number of the first response signals 51which the first communication unit 101 can receive per unit time periodand determine the limited time period on the basis of the resultantquotient.

In the present embodiment, the limited time period is determined as amaximum time period within which all the terminals 20B in the numberidentical to a difference between the total number of the terminals 20Bestimated by the terminal number determination unit 107B and the numberof the first response signals 51 counted by the counting unit 106B sendthe second response signal 52 to the controller 10B.

Accordingly, in the present embodiment, the above predetermined numberis the number of the terminals 20B which have not sent the firstresponse signal 51. For example, when the number of the first responsesignals 51 counted by the counting unit 106B is 10, and when the totalnumber of the terminals 20B determined by the terminal numberdetermination unit 107B is 100, and when the maximum number of the firstresponse signals 51 which the first communication unit 101 can receiveper unit time is 10, the limited time period is 9 [sec].

The limited time determination unit 110B provides the determined limitedtime period to the second request unit 111B.

The responding terminal storage unit 114 is configured to store theterminal identification information 502 of the first response signal 51received by the first communication unit 101. The responding terminalstorage unit 114 is configured to store the terminal identificationinformation 502 of the first response signal 51 received by the firstcommunication unit 101 within the predetermined time (reception time)from the time at which the first communication unit 101 has sent thefirst request signal 41. In other words, the responding terminal storageunit 14 creates a list of the terminals 20B which has responded to thefirst request signal 41.

The second request unit 111B is configured to send the second requestsignal 42B (see FIG. 9) to the plurality of the terminals 20B via thefirst communication unit 101 in response to receiving the limited timeperiod from the limited time determination unit 110.

The second request signal 42B includes a responding terminal list inaddition to the operation code 401, the interface identifier 402, theobject unique identifier 403, and the limited time information 406. Theresponding terminal list is defined as a list of the terminalidentification information 502 which has stored in the respondingterminal storage unit 114, that is, the terminal identificationinformation 502 of the terminal 20B which has sent the first responsesignal 51.

In brief, the controller 10B sends the second request signal 42Bincluding the responding terminal list 407 to the terminals 20B in orderto announce to the terminals 20B which terminal 20B has responded to thecontroller 10B.

The transmission check unit 207 is configured to, upon receiving thesecond request signal 42B via the second communication unit 201, checkwhether or not the responding terminal list 407 of the second requestsignal 42B contains the address information which is identical to theaddress information (terminal identification information) stored in thesecond communication unit 201. Upon acknowledging that the respondingterminal list 407 of the second request signal 42B contains no addressinformation identical to the address information stored in the secondcommunication unit 201, the transmission check unit 207 requests thewaiting time calculation module 2052B to calculate the waiting timeperiod. When acknowledging that the responding terminal list 407 of thesecond request signal 42B contains the address information identical tothe address information stored in the second communication unit 201, thetransmission check unit 207 does not request the waiting timecalculation module 2052B to calculate the waiting time period.

The waiting time calculation module 2052B requests the second randomnumber creation module 2051 to create the random number, upon beingrequested to calculate the waiting time period by the transmission checkunit 207. Upon receiving the random number from the second random numbercreation module 2051, the waiting time calculation module 2052Bcalculates the waiting time period.

As described in the above, when the terminal 20B acknowledges that itsaddress information is not included in the terminal list 407 (when theterminal 20B has not sent the first response signal 51 to the controller10B), the terminal 20B calculates the waiting time period. As a result,the terminal 20B sends the second request signal 52 to the controller10B. In brief, upon receiving the second request signal 42B, theterminal 20B checks whether or not it has sent the first request signal51 to the controller 10B. When the terminal 20B acknowledges that theterminal 20B has sent the first response signal 51 to the controller10B, the terminal 20B does not send the second response signal 52. Uponacknowledging that the terminal 20B has not sent the first responsesignal 51 to the controller 10B, the terminal 20B sends the secondresponse signal 52.

Next, an explanation is made to operation of the network system of thepresent embodiment.

In the controller 10B, upon receiving the request of sending the firstrequest signal 41, the first request unit 105B controls the firstcommunication unit 101 in a manner to send the first request signal 41to the plurality of the terminals 20B. For example, the first requestsignal 41 includes the operation code 401 of “Watch”, the object uniqueidentifier 403 of “*”, the interface identifier 402 of “IID7”, and theprobability information indicating the response probability of “10%”.

Only the terminal 20B including the interface having the interfaceidentifier “IID7” receives the first request signal 41. In the terminal20B which has received the first request signal 41, the first responseunit 204 controls the second communication unit 201 at the responseprobability indicated by the probability information 405 of the firstrequest signal 41. In other words, the first response signal 51 is sentto the controller 10B at the response probability indicated by theprobability information 405 of the first request signal 41.

As to the first response signal 51, the operation code 501 is “Provide”.The terminal identification information 502 denotes the address of theterminal 20B being the sender of the first response signal 51. Theparameter 503 is the combination of the interface identifier “IID7” andthe object unique identifier associated with the interface identifier“IID7”.

In the controller 10B, the counting unit 106B counts the first responsesignals 51 which the first communication unit 101 has received. Theterminal number determination unit 107B determines the total number ofthe terminals 20B having not responded to the first request signal 41 onthe basis of the number of the first response signals 51 obtained fromthe counting unit 106B and the response probability. When the tenterminals 20B send the first response signal 51, the first communicationunit 101 receives the ten first response signals 51. The responseprobability is “10%”. Thus, the terminal number determination unit 107Bdivides the number (=10) of the first response signals 51 received atthe first communication unit 101 by the response probability (=10%) toobtain a quotient (=1). The terminal number determination unit 107Bcalculates a difference (=90) between the number of the terminals 20Bderived from the quotient (=1) and the number (=10) of the firstresponse signals 51 received by the first communication unit 101. Theterminal number determination unit 107B provides the difference (=90) asthe total number of the terminals 20B.

The limited time determination unit 110B determines the limited timeperiod T10 on the basis of the total number of the terminals 20Bprovided from the terminal number determination unit 107B and notifiesthe second request unit 111B of the limited time period T10. When thetotal number of the terminals 20B determined by the terminal numberdetermination unit 107B is 90 and the maximum number of the firstresponse signals 51 which the first communication unit 101 can receiveper unit time is 10, the limited time period T10 is 9 [sec]. As apparentfrom the above, the limited time period T10 is shortened depending on anincrease in the number of the terminals 20B which has responded to thefirst request signal 41.

The responding terminal storage unit 114 stores the terminalidentification information 502 of the first response signal 51 which hasbeen received by the first communication unit 101.

Upon receiving the limited time period from the limited timedetermination unit 110B, the second request unit 111B controls the firstcommunication unit 101 in a manner to send the second request signal 42Bto the plurality of the terminals 20B. The second request signal 42Bincludes the same operation code 401 (=“Watch”), object uniqueidentifier 403 (=“*”), and interface identifier 402 (=“IID7”) as thoseof the first request signal 41. Further, the second request signal 42Bincludes the limited time information 406 and the responding terminallist 407. The limited time information 406 indicates the limited timeperiod determined by the limited time determination unit 110B, and theresponding terminal list 407 indicates the terminal identificationinformation stored in the responding terminal storage unit 114.

Only the terminal 20B including the interface designated by theinterface identifier “IID7” receives the second request signal 42B. Inthe terminal 20B which has received the second request signal 42B, thetransmission check unit 207 checks whether or not the respondingterminal list 407 of the second request signal 42B includes the terminalidentification information which is identical to the terminalidentification information stored in the second communication unit 201.Upon acknowledging that the responding terminal list 407 of the secondrequest signal 42B contains no address information identical to theaddress information stored in the second communication unit 201, thetransmission check unit 207 requests the waiting time calculation module2052B to calculate the waiting time period. Consequently, the waitingtime determination unit 205B determines the waiting time period on thebasis of the limited time period T10 indicated by the limited timeinformation of the second request signal 42B.

In this instance, the second response unit 206 controls the secondcommunication unit 201 after a lapse of the waiting time perioddetermined by the waiting time determination unit 205, thereby sendingthe second response signal 52 to the controller 10B. As to the secondresponse signal 52, the operation code 501 is “Provide”. The terminalidentification information 502 denotes the address of the terminal 20Bbeing the sender of the second response signal 52. The parameter 503 isthe combination of the interface identifier “IID7” and the object uniqueidentifier associated with the interface identifier “IID7”.

In contrast, when acknowledging that the responding terminal list 407 ofthe second request signal 42B contains the address information identicalto the address information stored in the second communication unit 201,the transmission check unit 207 does not request the waiting timecalculation module 2052B to calculate the waiting time period.Consequently, the waiting time determination unit 205B does notdetermine the waiting time period. In this instance, the terminal 20Bsends no second response signal 52 to the controller 10B.

According to the aforementioned network system of the presentembodiment, the limited time period is defined to be a predeterminedrange within which the first communication unit 101 receives thepredetermined number of the second response signals 52. Thepredetermined number is defined to be the number of the terminals 20Bwhich have not sent the first response signal 51. The limited timedetermination unit 110B is configured to divide a difference by thenumber of the first response signals 51 which the first communicationunit 101 can receive per unit time period to obtain the limited timeperiod. The difference is defined as a difference between the totalnumber of the terminals 20B determined by the terminal numberdetermination unit 107B and the number of the first response signals 51counted by the counting unit 106B. Each of the terminals 20B isconfigured to send no second response signal 52 once after sending thefirst response signal 51 to the controller 10B.

According to the network system of the present embodiment, the terminal20B once responded to the first response signal 41 will not respond tothe second request signal 42B. Therefore, the terminal 20B responds onlyto either one of the first and second request signals 41 and 42B. Thus,the limited time period can be shortened in contrast to an instancewhere each of the terminals 20B sends the second response signal 52irrespective of whether or not it has responded to the first requestsignal 41. Consequently, the communication efficiency can be improved.

Besides, the network system of the first or second embodiments mayinclude a plurality of the controllers 10, 10A, or 10B. Further, thecontroller 10, 10A, or 10B may broadcast the request signal.

1. A network system comprising: a controller; and a plurality ofterminals connected to said controller via a network, wherein saidcontroller comprises a first communication unit configured tocommunicate with said terminal via the network, a first request unit, acounting unit, a terminal number determination unit, a limited timedetermination unit, and a second request unit, each of said terminalscomprising a second communication unit configured to communicate withsaid controller via the network, a first response unit, a waiting timedetermination unit, and a second response unit, said first request unitbeing configured to send a first request signal to the plurality of saidterminals via said first communication unit, the first request signalincluding probability information indicative of a predeterminedprobability, said first response unit being configured to, uponreceiving the first request signal via said second communication unit,send a first response signal to said controller via said secondcommunication unit at the probability indicated by the probabilityinformation included in the first request signal, said counting unitbeing configured to count the first response signals received via saidfirst communication unit, and output the obtained number of the firstresponse signals, said terminal number determination unit beingconfigured to determine a total number of said terminals responding tosaid controller on the basis of the probability and the number of thefirst response signals obtained from said counting unit, said limitedtime determination unit being configured to determine a limited timeperiod on the basis of the total number of said terminals determined bysaid terminal number determination unit, said second request unit beingconfigured to send a second request signal to the plurality of saidterminals via said first communication unit, the second request signalincluding limited time information indicative of the limited time perioddetermined by said limited time determination unit, said waiting timedetermination unit being configured to, upon receiving the secondrequest signal via said second communication unit, select a waiting timeperiod in a random manner from time periods not greater than the limitedtime period indicated by the limited time information included in thesecond request signal, and said second response unit being configured tosend a second response signal to said controller via said secondcommunication unit after a lapse of the waiting time period.
 2. Thenetwork system as set forth in claim 1, wherein said controller furthercomprises a storing means configured to store information derived fromthe number of the first response signals, said controller beingconfigured to use the information stored in said storing means to createthe second request signal and send the created second request signal. 3.The network system as set forth in claim 2, wherein said storing meansis defined as a terminal number storage unit configured to store thetotal number of said terminals determined by said terminal numberdetermination unit as the information derived from the number of thefirst response signals, said limited time determination unit beingconfigured to determine the limited time period by use of the totalnumber of said terminals stored in said terminal number storage unit. 4.The network system as set forth in claim 2, wherein said storing meansis defined as a limited time storage unit configured to store thelimited time period determined by said limited time determination unitas the information derived from the number of the first responsesignals, said second request unit being configured to create the secondrequest signal which includes the limited time information indicative ofthe limited time period stored in said limited time storage unit.
 5. Thenetwork system as set forth in claim 3, wherein said controller furthercomprises a terminal number update unit, said first request unit beingconfigured to send the first request signal to the plurality of saidterminals periodically via said first communication unit, and saidterminal number update unit being configured to, upon acknowledging thatsaid terminal number determination unit newly determines the totalnumber of said terminals, update the total number of said terminalsstored in said terminal number storage unit in match with the totalnumber of said terminals newly determined by said terminal numberdetermination unit.
 6. The network system as set forth in claim 4,wherein said controller further comprises a limited time update unit,said first request unit being configured to send the first requestsignal to the plurality of said terminals periodically via said firstcommunication unit, and said limited time update unit being configuredto, upon acknowledging that said limited time determination unit newlydetermines the limited time period, update the limited time periodstored in said limited time storage unit in match with the limited timeperiod newly determined by said limited time determination unit.
 7. Thenetwork system as set forth in claim 1, wherein the probabilityindicated by the probability information is determined on the basis of amaximum number of the first response signals which the firstcommunication unit can receive simultaneously, so as to allow said firstcommunication unit to receive the first response signals respectivelyfrom all of said terminals within a predetermined time period startingfrom the time of sending the first request signal.
 8. The network systemas set forth in claim 1, wherein said terminal number determination unitis configured to determine the total number of said terminals on thebasis of a quotient of the number of the first response signals obtainedfrom said counting unit and the probability of the probabilityinformation included in the first request signal.
 9. The network systemas set forth in claim 1, wherein the limited time period is defined tobe a predetermined range within which said first communication unitreceives a predetermined number of the second response signals, thepredetermined number being defined to be the total number of saidterminals determined by said terminal number determination unit, andsaid limited time determination unit being configured to calculate aquotient of the total number of said terminals determined by saidterminal number determination unit and the number of the first responsesignals which said first communication unit can receive per unit timeperiod and determine the limited time period on the basis of theresultant quotient.
 10. The network system as set forth in claim 1,wherein the limited time period is defined to be a predetermined rangewithin which said first communication unit receives a predeterminednumber of the second response signals, the predetermined number beingdefined to be the number of said terminals which have not sent the firstresponse signal, said limited time determination unit being configuredto divide a difference by the number of the first response signals whichsaid first communication unit can receive per unit time period todetermine the limited time period, the difference being defined as adifference between the total number of said terminals determined by saidterminal number determination unit and the number of the first responsesignals counted by said counting unit, and each of said terminals beingconfigured to send no second response signal once after sending thefirst response signal to said controller.